Manufacturers of semiconductor chip packages are constantly striving to develop smaller chip packages with enhanced capabilities. For example, flip chip technology has developed as a result of the movement toward ever increasing miniaturization of electronic circuitry. This technology is also known as direct chip attach or "DCA". It includes "flip chip" bonding, wherein a flip chip integrated circuit (IC) is electrically and mechanically connected to a substrate through solder balls on the underside of the flip chip that are registered or aligned with solder pads on the substrate.
Generally referring to FIGS. 1-3, it is well known that a semiconductor die or flip chip 10 is provided with a pattern of solder bumps or balls 12 on an underside or circuit side thereof. The solder balls 12 are registered or aligned with solder pads 14 on a PC board or similar substrate 16. The underside of the chip 10 is also referred to as the image side of the chip. Flux (not shown) is normally supplied between the solder balls 12 and solder pads 14. Upon heating, the solder pads 14 on the PC board or substrate 16 reflow and physically connect with the solder balls 12 on the underside of the chip 10. The solder balls 12 typically have a high melting point and therefore do not reflow. This connection is illustrated diagrammatically in FIG. 2 by deformed solder pad 14' mating with a solder ball 12. This process eliminates the requirement for wire bonding.
Special liquid epoxy 18 (FIG. 3) is typically used to completely fill the underside of the chip. This is referred to herein as the "underfill" operation. Upon curing, the resulting encapsulation forms a non-hygroscopic barrier to prevent moisture from contacting and thus corroding the electrical interconnects between the PC board 16 and the chip 10. The epoxy 18 also serves to protect the bonds between the deformed solder pads 14' and the solder balls 12 by providing thermal stress relief, i.e., accommodating different rates of thermal expansion and contraction.
Referring to FIG. 3 of the drawings, once the underfill operation is complete, it is desirable that enough liquid epoxy be deposited along the edges of the chip 10 to fully encapsulate all of the electrical interconnections and so that a fillet 18a is formed along the side edges of the chip 10. Normally, the liquid epoxy flows under the chip 10 as a result of capillary action due to the small gap between the underside of the chip 10 and the upper surface of the PC board or substrate 16. As the surface area of the chip 10 increases and/or the gap 15 between the chip 10 and the substrate 16 becomes smaller, it becomes more difficult and time consuming to fully encapsulate all of the electrical interconnections. This may result in incomplete underfill wherein voids 20 in the gap 15 may exist. If such voids are present, then corrosion and undesirable thermal stresses may result which may result in deceased performance or early failures.
It would therefore be desirable to provide a manner of underfilling the gap formed between a flip chip and a substrate, and especially in applications involving larger flip chip geometries and smaller gaps, while preventing any voids or spaces left unfilled between the flip chip and the substrate.